1. Field of the Invention
The present invention involves magnetic resonance imaging (MRI) technology, and more particularly a method and an apparatus for accelerating the magnetic resonance (MR) temperature imaging.
2. Description of the Prior Art
In an MR-monitored high intensity focused ultrasound (HIFU) therapy system, a HIFU transducer, i.e. HIFU therapy head, emits focused ultrasonic waves into the human body while through therapy water to increase the local temperature of the body part to be treated, hence achieving the desired therapeutic results.
There are three issues inherent with MR temperature imaging: temporal averaging effect, temporal resolution and spatial averaging effect. These issues must be fully considered in the design of an acceleration solution for MR temperature imaging.
Usually, it takes a long time to acquire data to be entered into k-space and the acquisition of data sufficient to generate only one MR image may take several seconds. During the data sampling, the temperature around the heating focus will change continuously, so the temperature is changing continuously while data entered into different points of k-space are acquired. The final temperatures obtained from the reconstructed images are approximations of all the actual temperatures in the course of sampling, which is referred to as the temporal averaging effect. FIG. 1 is an illustration of the temporal averaging effect. The round spot shown in FIG. 1 is the temperature value measured from the rebuilt images, and the curve is the plotting of temperature changes over the acquisition time of one image. Typically, the measured value is closer to the value acquired at the center of k-space.
In order to improve spatial resolution while maintaining the size of the FOV (field of view), more phase encoding steps are needed, leading to more time for acquiring all data of k-space. The temporal resolution is defined as a reciprocal of the time span for acquiring the centers of two sequential k-space matrices. The longer it takes to fill a single k-space matrix, the poorer the temporal resolution is, making it difficult to capture the fast changes of temperature. FIG. 2 shows the impact of temporal resolution on capturing the temperature changes. The curves shown in the FIG. 2 represent the temperature changes and the round spot represents sampling points. Referring to FIG. 2, the temporal resolution shown in the left curve is relatively low and so the data of peak temperature values are lost due to the low temporal resolution while in the right curve, the peak temperature value is acquired as a result of the high temporal resolution.
Each pixel of the MR image represents a voxel of a certain size of the object. The signals of a pixel are the sum or integral of multiple tiny signals in a corresponding voxel. In thermal ablation, if the size of a single voxel is approximately the size of the heating focus, the spatial gradient of temperature will be significant, resulting in the elimination of phases, which in turn leads to distortion of temperature measurement. FIG. 3 is a schematic illustration of the temporal averaging effect. At the left of FIG. 3, the voxels at the focuses are indicated by the arrows. In the middle of FIG. 3 the signal of pixels corresponding to the focus in the rebuilt images is shown. At the right of FIG. 3, shows the actual signal of the focus is shown. As shown in FIG. 3, because the pixel size is only slightly larger than the size of the focus of the ultrasound, there is a significant difference between the signals of the corresponding pixels in the rebuilt images and the actual signals of the ultrasonic focus. The spatial resolution is defined as the reciprocal of voxel dimension. There is a need to improve the spatial resolution of accurate temperature imaging.
Generally speaking, temporal resolution and spatial resolution contradict each other and it is impossible to improve temporal resolution and spatial resolution at the same time. For a given FOV, the smaller spatial resolution requires a longer acquisition time, making the temporal resolution worse not better. Therefore, it is necessary to balance these two parameters.
With existing several parallel imaging technologies, acceleration is achieved by regularly reducing the phase encoding lines and then recovering the lost data with post processing, to improve either temporal resolution or spatial resolution. It is impossible to have a better temporal resolution and a better spatial resolution at the same time. These methods have two drawbacks. First, the acceleration rates achieved by these parallel imaging technologies are usually relatively fixed discrete values and changes of acceleration factors will cause significant fluctuation in image quality. A second problem is instability, namely, misalignment of coil units for parallel imaging or improper positioning of the object may cause serious residual artifacts in the rebuilt images and covering of the heating focus by the residual artifacts will cause significant errors in calculation of temperatures, leading to very serious consequences of the HIFU therapy.